As is known, optical pulsed radar, equipment must measure ranges by using the delay time principle. The time interval between the transmission of a transmitted signal and the reception of an echo signal is used as a measure of the distance from the transmitter-receiver to the target. Between those two signals, pulses supplied to an evaluation circuit by a crystal oscillator, commonly a quartz oscillator having a fixed frequency, are counted. The transmitted signal, or a pulse occurring with the transmitted signal, is used as a start signal and the received or echo signal is used as a stop signal for the control of the evaluation circuit. Details of such an apparatus can be found in the article "Measuring Short Ranges with the Aid of Optical Pulsed Radar Equipment," Nachrichtentechnische Zeitschrift, 1973, No. 9, pp. 535 to 540. This technique has the difficulty that there is no correlation between the transmitted signal (or the start signal) which starts the pulse counting and the leading edges of the pulses of the quartz oscillator, nor between the received signal (echo signal) which is used to end the pulse counting and the pulse fronts of the oscillator. Thus, at the beginning and end of the pulse counting, there are residual times which are not taken into consideration and are not evaluated for range measurement purposes. Each of these undeterminec residual times is smaller than the interval between two clock pulses. It is quite acceptable to ignore those residual times when accomplishing rather coarse range measurements involving large numbers of meters or kilometers. However, a technique which ignores these residual times is inadequately accurate for fine range measurements requiring accuracies in the order of millimeters and centimeters.
German OS No. 2,842,450 and U.S. Pat. No. 3,541,448 disclose methods and devices which, in a coarse measurement, take account of the range and, in an additional fine measurement, take account of the residual times. The precise range is then derived from those two measurements. However, such apparatus has the disadvantage of being very expensive. The known devices are also too heavy and large and therefore unsuitable for mobile use as, for example, a range finder in geodesy. It is not possible to achieve adequate miniaturization with the technical means based on the known devices and methods.